The present invention generally relates to electron beam exposure systems and more particularly to an electron beam exposure system for writing a number of patterns on an object consecutively with increased efficiency.
The electron beam exposure system is used for writing submicron patterns on an object such as a semiconductor wafer. In using the electron beam exposure system for mass-producing semiconductor devices having submicron patterns such as LSIs, it is essential that the electron beam exposure system has a large throughput. It should be noted that, as a result of the submicron patterning, enormous numbers of semiconductor devices are formed on the substrate of the LSI. On the other hand, the usual electron beam exposure system generally has a limited throughput originating from the principle of the electron beam exposure that the pattern is written by a single electron beam. In other words, the semiconductor patterns are written on the substrate in one stroke.
In order to achieve the desired high throughput, a technique called a block exposure process is proposed. According to this technique, the electron beam is passed through a suitable electron optic system to form a parallel beam, and the parallel beam thus obtained is passed through an apertured mask for beam shaping. The mask carries thereon a number of fundamental patterns of semiconductor devices in the form of apertures and shapes the electron beam upon passage therethrough. Further, in order to address the desired aperture, the electron beam exposure system includes a deflection system for deflecting the parallel electron beam.
FIG. 1 shows the principle of the block exposure.
Referring to FIG. 1, an electron gun 1 produces an electron beam that is directed to a semiconductor substrate 7 along a predetermined optical axis. Thereby, the electron beam is first passed through a shaping aperture 2 for preliminary shaping and focused on a point located on the optical axis by an electron lens 3. There, the electron beam is deflected by a deflector 4 and hits a predetermined part of a mask 5 that carries a number of apertures 6a-6c corresponding to the fundamental patterns of semiconductor devices. By controlling the deflector 4, one can deflect the electron beam to hit any desired aperture on the mask 5. In other words, one can address the apertures on the mask 5 by the deflector 4.
As a result of the foregoing addressing, the electron beam passes through the mask 5 while being shaped according to the shape of the aperture through which the electron beam has passed. After the shaping thus occurred, the electron beam is deflected back on the optical axis by an electron lens 8 that is formed to surround the mask 5. Further, the electron beam is focused on the surface of the substrate 7 after passing through a demagnification electron optical system 9. Thereby, the demagnified pattern of the aperture on the mask 5 through which the electron beam has passed is exposed on an electron beam resist (not shown) provided on the surface of the substrate 7. According to such a block exposure technique, one can reduce the exposure time significantly as compared to the case for writing each pattern by a finely focused electron beam.
In carrying out the foregoing block exposure, it is necessary to address the apertures on the mask 5 consecutively. On the mask 5, about several hundreds to several thousands of such apertures are formed, and an enormous number of semiconductor devices is exposed on the single substrate 7. Thereby, mere implementation of the block exposure process is not sufficient to achieve the desired throughput. It should be noted that such a simple block exposure process includes the addressing of the apertures on the mask 5 for each exposure on the substrate 7.
In the block exposure process, there occurs frequently the case wherein one aperture pattern is written on a number of different locations on the substrate 7 consecutively. Thus, there is a conventional process to reduce the time needed for the exposure by compressing exposure data for controlling the exposure process, by dividing the exposure data into first type data specifying the apertures on the mask and second type data specifying the number of exposures to be repeated.
FIG. 2 shows a conventional process for implementing the foregoing data compression.
Referring to FIG. 2, the exposure data is held in a storage device 110 and read out therefrom consecutively. The exposure data thus read out is then stored in a FIFO memory 111 and transferred further to a first register 112 therefrom.
Now, the exposure data includes two types of data, first type data P for designating the aperture or pattern, and second type data M for designating the number of repetitions of the exposure, as described above. In order to identify the attribute of the first and second data, the data P and data M carry respective identification flags in a suitable bit thereof.
Once the exposure data that is either of the data P or data M is stored in the register 112, the identification flag is detected by a discrimination part 113, and the discrimination part 113 controls a switch device 114 that is connected to an output port of the register 112 for transferring the data P and the data M respectively to buffer registers 115 and 116. Based upon the data P outputted by the register 115, the addressing of the apertures on the mask 5 is achieved, while the data M is used for moving the substrate 7 and for deflecting the shaped electron beam over the surface of the substrate 7 for repetitive exposure.
In the conventional data compression system of FIG. 2, the data compression occurs as follows.
Assuming a series of exposure data coming in as . . . P.sub.4 /P.sub.3 /P.sub.2 /M.sub.2 /P.sub.1 /M.sub.1 wherein the data M.sub.1 comes in first, the content of the registers 115 and 116 changes with time t.sub.1, t.sub.2, t.sub.3, . . . as follows.
TABLE I ______________________________________ t6 t5 t4 t3 t2 t1 115 115 115 116 115 116 P.sub.4 P.sub.3 P.sub.2 M.sub.2 P.sub.1 M.sub.1 ______________________________________
As can be seen in the above transition table, the storage of the data P and M into respective registers 115 and 116 is achieved alternately in response to the alternate arrival of the data M and data P. This means that the state of the register 115 remains unchanged when the data M is transferred to the register 116, and the state of the register 116 remains unchanged when the data P is transferred to the register 115. Thus, it will be understood that there exists a loss of time in the operation of the conventional system of FIG. 2. This loss becomes particularly conspicuous when a semiconductor pattern having a large number of repetitions is exposed.